There is a known technology of acquiring a road image in front of a vehicle with a camera mounted on the vehicle, detecting a driving lane on which the host vehicle is driving, by image processing, determining a possibility that the host vehicle will deviate from the driving lane, based on the detected driving lane information and an estimated path of the host vehicle, and, with a high possibility of a deviation, issuing a warning to a driver to prompt avoidance of the deviation by a steering operation or the like (e.g., cf. Japanese Patent Application Laid-Open No. 7-105498).
This technology is, for example, to determine a point of the deviation from the driving lane, based on an offset, a yaw angle, a curve radius, etc. of the driving lane and a yaw rate, a steering angle, a vehicle speed, etc. of the host vehicle, and to predict a state of the deviation on the basis of a distance between the foregoing point and the host vehicle and an angle between the estimated path at the point and a partition line of the driving lane.
In the foregoing technology, a deviation warning is issued when the distance between the predicted deviation point and the host vehicle is smaller than a distance threshold set nearly in proportion to the vehicle speed. As this distance threshold is set nearly in proportion to the vehicle speed, a predicted arrival time to the predicted deviation point (deviation prediction time) is kept approximately constant to guarantee a margin time for execution of a deviation avoidance operation.